1. Field of the Invention
This invention relates to a method of conducting a chemical reaction comprising the oxidation of carbon monoxide to carbon dioxide employing a catalyst, to an engine whose exhaust apparatus contains the catalyst in order to combat air pollution, to a certain such catalyst containing ceria and its preparation and use, and to the preparation of ceria.
2. Background Discussion
Transition metal catalysts, especially those involving Group VIII metals, are usually prepared by dispersing the active component on high surface area carriers, such as refractory inorganic oxides, to achieve maximum specific metal surface areas and high thermal stability. The support material must usually have a high surface area, and various parameters are considered in selecting a support for metal catalysts. Metal oxides are the most commonly used carder, and their dominance is a consequence of their generally high thermal and chemical stability, coupled with the knowledge of preparation of the materials with high surface areas. Support materials can be classified into either inert supports, like SiO.sub.2, supplying high surface area for dispersing the active component, or catalytically active supports, like SiO.sub.2 --Al.sub.2 O.sub.3 or zeolite, for bifunctional catalysts. The nature of the carder oxide can affect the size and morphology of the metal particles, either during deposition or activation, an effect generally referred to as non-specific metal-metal oxide interaction. In certain other cases, the support can influence the active component by a strong interaction, an example being partially reduced TiO.sub.2. Support for the view that the metal oxide support can significantly influence the catalytic properties of the metal in a more specific manner was provided by Tauster and co-workers (U.S. Pat. No. 4,149,998) who described the unusual effects observed in certain metal-metal oxide systems and introduced the acronym SMSI (strong metal support interaction). The SMSI phenomenon, caused by the high temperature reduction of Group VIII metals dispersed on certain metal oxides, is characterised by a suppression in the metal's ability to chemisorb CO or H.sub.2, increased catalytic activity in CO/H.sub.2 reactions, and decreased activity for structure sensitive reactions such as alkane hydrogenolysis, and is reversible in the sense that mild oxidation will restore the catalyst to its original state. The dispersion of the metal particles, thus the metal-metal oxide interfacial area, and the temperature of H.sub.2 reduction will both influence the extent of the SMSI effect. The various hypotheses invoked to explain the SMSI phenomenon include explanations involving a geometric effect caused by migration of oxidic moieties from the support to the surface of the metal particles, an electronic effect caused by charge transfer from cations on the oxide surface to the metal, or the creation of new active sites at the metal-metal oxide interface.
The concept that a H.sub.2 reduction pre-treatment could influence catalytic activities of metals supported on metal oxides to enhance the low temperature CO oxidation activities of certain metal-metal oxides catalysts has been described in various patent specifications, for instance European patent specification 0337446. These do not specify whether the induced changes are due to a SMSI-type effect or the simple reduction of the metal component. The advent of automobile exhaust catalysts has led to further intensive investigations of the interaction of noble metals with metal oxides, notably CeO.sub.2. Yu Yao (J of Catalysis, 87, 152-162 (1984)) has shown that treatment of Pd, Pt or Rh catalysts supported on CeO.sub.2 --Al.sub.2 O.sub.3 under reducing conditions results in a dramatic, but transient, enhancement of catalytic activity for the oxidation of CO and hydrocarbons. Other research groups have studied specifically the Pt--CeO.sub.2 interaction after reduction pre-treatment and have shown enhancement in conversion activity for CO oxidation and NO.sub.x reduction. The degree of the Pt--CeO.sub.2 interaction was dependent on both the Pt and CeO.sub.2 crystallite sizes. The catalysts were designed to achieve highly dispersed Pt on high surface area CeO.sub.2 (for high Pt--CeO.sub.2 interfacial area) by impregnation techniques, prior to the reduction pre-treatment.
The strong metal-metal oxide interaction described in this prior literature has been observed on catalysts prepared by conventional impregnation techniques. Such procedures do not lead to a material with intrinsically unique or unusual catalytic properties. The treatment of these systems in a reducing atmosphere at high temperature induces the reported unique properties and activities of these catalysts. In addition, many of these systems are reversible in that a subsequent treatment in an oxidising atmosphere at a certain temperature, normally greater than 200.degree. C., negates these unique catalytic properties and therefore reverses any changes in the structure or electronic character which account for the unique catalytic properties and can severely impair the usefulness of such catalyst systems.